Magnetron oscillator and modulation means therefor



' S. LINK Nov. 12, 1940.

MAGNETRON oscmm'ron AND MODULATION mmus THEREFOR 2 Sheets-Sheet 1 Filed Jan. 21, 1938 ne/fc R E QW VN n ma K Y B Nov. 12, 1940. K 2,220,968

MAGNETRON OSCILLATOR ANb MODJLATION MEANS THEREFOR Filed Jan. 21. 1938 2 Sheets-Sheet z I mxynr-{IMWI Z6 24 I 134 i 32 27 25 I I I 'rmu6m {|||l 3/ l g AMPLIFIER 1 1 .9 v MAGNET/C W f 40 co/z P AMPL/F/EK I lllllll ll uopuu r/o m F 9 10 SOURCE 22 Mom/um SOURCE AG/VFT/C 4 F/Ew COIL g 33 23 a I I I1 2 42 Ill 43 AMPLIFIER l l l l l l 46 01 I]! I V II I, v I" V.

I I! 47 I 1 I I INVENTOR' SIMON LINK ATTORNEY Patented Nov. 12, 1940 MAGNET RON OSCILLATOR AND MODULA- TION MEANS THEREFOR Simon Link, Berlin-Spandau, Germanyyassignor-to Siemens '& Halske, Aktiengesellschaft,

Siemensstadt, near Berlin, Germany, a corporation of Germany Application January 21, 1938, Serial No. 186,042

Germany January 21, 1937 v 4Claims.

ed for modulation of the high frequency oscillations generated by a magnetron type of discharge tube. With a view to insuring maximum efii- 5 ciency it would be desirable to fully modulate the power delivered by the tube, with complete abstraction of power. In the operation ofheretofore known systems, however, it has .been found that discontinuity effects occur whenever attempts have been made to modulate the oscillations produced by a magnetron tube of conventional design. In the customary methodofapplying modulation potentials to the anodes of a magnetron discharge tube the control upon the emission current has been found to be quite limited. In other words, only a low modulation percentage is obtainable because when the amplitudes of the modulation exceed a certaincritical value oscillations at the high frequency, are broken off. But if the delivered power is diminished in order that a more favorable modulation range may be secured, this will occasion again an undesirable decrease in the efliciency of the tube,

and this means that-improvement in the modulation range is only obtainable by a sacrifice of t the overall efiiciency of the system.

Accordingly, it is an object of my invention to provide apparatus and the method for modulating a magnetron oscillator tube so as to deliver modulated, ultra-high frequency power at optimum efficiency. It is a further object of my invention to arrange the circuits of the magnetron discharge tube oscillator so that oscillations will not be interrupted by applying substantially 100% modulation potentials thereto.

According to the invention, the values assigned to the various components of the circuit arrangement and to the potential sources applied thereto are varied in such a manner that a constant negative resistance characteristic .is maintained in the operation of the magnetron tube, notwithstanding the impress of modulation potentials upon the circuit thereof. The new results thus obtained are made possible by virtue of the 4 fact that symmetric. modulation potentials with respect to a median value are impressed upon the anode segments so that the voltage distribution v .of the ultra-high frequency currents generated will be suitable for maintaining the oscillations 50 in the magnetron tube. The voltage difference compared with the median value is equivalent to the amplitude of the alternatin potential. That is,one anode segment is as much negative in ref-' erence to the mean value as the other is positive. 55 The voltage of the median value is merely that (01. 179-1715) This inventionrelates to a system and a methwhich -polarizes the anodes positively with respect to the cathode in order to produce electronic emission.

Figures 1 to 7 inclusive and Fig. 11 show various characteristic 'curves'which are hereinafter 5 referred to in explaining the theory of operation of my invention and in contrasting the same with systems of the prior art;

Fig. 8 is a circuit diagram of a preferred embodiment of my invention; and 10 Figs. 9 and 10 show respectively one and another modification of the circuit arrangement of Referring to Fig. 1, this diagram shows the anode current as a function of the applied potenl5 tial in the absence of a magnetic field. Curve portion l represents the current in'one anode segment, while portion 2 corresponds to'the current in the respective other segment, both as a function of the anode segment potentials E1 and 0 l'lz, respectively. .The abscissa M represents the mean anode potential above and below which the potentials E1 and E2 are varied. These anode potentials have a value which is determined by I a steady current from any suitable source con- 25 nected between the cathode and the anode segments and by the superposing 'of .a variable-modulation potential such as is caused to produce simultaneously a more positive potential upon one of the anode segments while the potential ap- 30 plied to the other anode segment is rendered more negative.

-The current distribution curvel ig. 1 (which holds good as long as no magnetic field operates),

becomes distorted as soon as a magnetic field is 35 applied to the zone of electronic emission. The curve of current distribution then presents, a more or less drooping or rising characteristic. The segment presenting decreasing potential receives a current which rises to a maximum or.

crest value according to slope 3, Fig; 2. crest value of the current shifts in the direction of lower segment potential as the magnetic field is increased in intensity. Furthermore, its absolute size decreases. This fact will be appreciated by a comparison between slope 3, Fig. 2, and slope 4, Fig. 3. For the segment with increasing positive potential above the mean value M the current initiallyg'rows slowly, but later at a very rapid rate up to the total saturation current of the cathode (slope 5, Fig. 2, and slope 6, Fig. 3); The diiference'betweena magnetron tube working with a low magnetic field, and a magnetron operating with a larger magnetic field, is clearly evidenced from a comparison of Figs. 2 and 3.

It will be seen that the distinction primarily resides in the fact that for the anode segment with decreasing potential the current crest for a smaller magnetic field lies at a higher segment potential and assumes a higher value, whereas for a larger magnetic field the current crest has a lower value and lies-in a different place, that netic field. It will be noted that up to certain,

voltage difl'erences compared with the median value, the characteristic is negative, and the negative range of the characteristic is limited by the current maximum. For greater difierences or deviations of the potential in comparison with the median value, the characteristic becomes positive again. The slope of the negative portion of the characteristic, as roughlydndicated by the straight lines l4 and I5, is of different values for different magnetic field intensities, as shown in Fig. 4. Thus, the negative resistance does not stay constant. The amplitude of the alternating potential l3 also changes in accordance with the position of the crests of the current.

From this shape of the push-pull characteristic, it is possible to get an idea regarding the conditions prevailing in the oscillatorystage of operation. Associating with the anode segments a resonant system, the latter will be caused to start oscillating by virtue of the negative resists 40 ance characteristic. If the drooping part of the characteristic were unlimited, then, in the .absence of all external load, that is, of damping, the amplitude of the alternating potential would grow ad infinitum. However, because of the re- 45 versal of the characteristic in the positive sense,

a limitation is imposed upon the amplitude, and

the same will be so adjusted that the negative damping of the decreasing portion of the characteristic is neutralized. Now, another positive 50 damping is introduced into the oscillatory system by the external load. Hence, the alternating voltage is thus reduced a corresponding amount, A

limit is reached when the positive damping due to the load happens to correspond to the-negative 55 damping of the characteristic, that is, when the load resistance becomes equal to the negative resistance defined and governed by the slope of the decreasing portion of the characteristic. I

Now, if in line with previous practice, for the 60 purpose of modulation, the magnetic field orthe potential is varied, then the crest of the current will change its position in respect of the median voltage value without,- however, incidentally altering to any. appreciable extent its absolute mag- 65 nitude.

istic, and this is an essential and decisive feature. If, then, an attempt is made to'extend the modulation range, in the presence of full or 'at least 70 comparatively large powe'r absorption, there will always be discovered places in the modulation process where the absolute value of the negative resistance of the tube becomes lower than the re-' ,sistance which is due to the load. At such in- 75 stant, that is, when the associated resistance pre- But this entails also a change, in the slope of the drooping portion of the charactervails over the efiect due to the negative tube resistance, the tube is no longer oscillable, in fact, the oscillation is broken off. If by some suitable means control of the current is efiected, the result, to be sure, will be that the current crest stays unvariedin its position in reference to the median value of the potential; but it will change essentially its amplitude. And also this will again cause a change in the slope of the negative portion of the characteristic, and as a result discontinuity or break-off eflects will be inevitable.

This is demonstrated in Fig. 5 which shows a characteristic covering a typical case where variable modulation currents are applied. The slope of the negative characteristic for current curve [6 is indicated by the straight line 517. If, then, the size of the current is altered as shown by curve It, then the inclination of the negative portion of the characteristic, as indicated by the straight line 69, is likewise altered. In other words, for full modulation, conditions will be found where the negative tube resistance is too low compared with the associated or coupled lead resistance, and where the tube is no longer oscillatory. Consequently, .the oscillations break off.

Now, by the method here disclosed these disadvantages are obviated. The starting and basic consideration is that the characteristic will not change if the anode segments as above set forth for the static case are impressed in the oscillatory state with difierent potentials which are symmetric to a median value so that the slope of the negative portion of the characteristic will stay constant even when the diflerence of the voltages is varied. With an increase in the voltage deviation the deliverable high frequency power will decrease, and the same may be steadily cut down to zero.

The operating conditions prevailing in this instance may be seen from Fig. '6. This diagram again shows the characteristic 20 ofa magnetron tube and the amplitude of the alternating potential 2! in the presence of maximum energy absorption. It should be noted from this diagram that the voltage value of the high frequency current is a function of the potentials applied to the anode segments, and that the departures of these potentials from a median value are symmetric. It is also evident that the potential amplitude will be lower or greater, depending upon whether the voltage difierence Vm is greater or smaller. On the one hand, this arrangement facilitates modulation with complete energy abstraction while, on the other hand, it allows for optimum adjustment of the working point along the modulation curve.

Fig. 7 illustrates the method of modulation last mentioned. As a relative measure for the out put the current I of an associated diode is indicated by the ordinates and the difference of the segment potentials Vm is given by the abscissae. The working point on the characteristic must be chosen according to the particular value obtained for the said deviation or difference. superposed upon the potentials of the anode segments, say, by the intermediary of a push-pull transformer, are the modulation potentials (alternating potentials). The mean value of the segment potentials, therefore, does not change, and the slope of the negative portion of the characteristic which governs the ability of the tube tooscillate and the prevention of discontinuity or rupture effects, remains permanently constant,

A practical instance for a suitable circuit 'organization adapted to practice the modulation method hereinbefore disclosed is shown in Fig. 8. The anode segments 22 and 23 are impressed with potentials which are symmetric to a median value and which are derived from batteries 24 and 25 by way of the choke-coils 26 and 21. Cathode 28 of the magnetron tube is united with the midpoint 29 of the secondary winding 30 of the push-pull transformer 3|, The modulation potential is thus symmetrically superposed in push-pull upon 10 the anode segment potentials, The primary windi5 power is taken oi the oscillatory circuit by a ing 32 of the push-pull transformer 3| is fed from the modulation amplifier 33. The tuning of the oscillatory system is accomplished by the variable condenser 34. The modulated high frequency dipole 35 in coupling relation withit.

A modification of the circuit organization as hereinbefore described is shown in Fig. 9. This arrangement corresponds in some respects to Fig.

20 8. The voltage derived from the battery 36 is symmetrically divided between the two anode segments. The modulation. potential furnished from the modulation amplifier 33 is symmetrically applied through the primary coil 38 of the :5 modulation transformer 39 to the two secondary coils 40 and 4! leading to the two anode segments 22 and 23, respectively.

The circuit organization could also be of a form as shown in Fig. 10. This scheme distinzo guishes itself from that in Figs. 8 and 9 by the provision of the battery 42 and 43, and also by virtue of the fact that the secondary coil of the modulation transformer is split into two windings 44 and 45.

5 In the operation of the invention it is important ---.tem. For instance, the anode current may be altered by variation of the heating or cathode to control the slope of the negative part of the characteristic and thus to avoid discontinuity of the oscillations when varying several electrical quantities as necessitated by the modulation syscurrent, or by the use of other arrangements for the control of the anode segment current; and

this in turn could be combined with a variation of o variation of other electrical data. This cndi-' the magnetic field intensity or of the aggregate anode potential or of a segment potential with the consequence that the variation of the negative part. of the characteristic occasioned by the control of the anode current is compensated by the tion is illustrated in Fig. 11. Referring to the characteristic denoted by reference numeral 46, the current crest according to curve 41 is displaced by an alteration of'the magnetic field,

5 and if no other steps be adopted, this results not only in a change in the value of the oscillatory potential, but also in a change of the slope of the negative characteristic; But if at the .same time by current control action the maximum or crest v of the current is raised in accordance with curve 0 48, then the slope of the negative portion of the characteristic stays stable and constant. It will thus be seen that discontinuity or rupturing effects may be avoided by simultaneous and joint control both of the current and of the strength of the magnetic fiel Also by a convenient change in'the magnetic field intensity and the anode potential alone, it

will be possible to insure the desired results.

If modulation is brought about by laterally mounted plates which set up,an additional electrical field in the direction of thelines of force of the magnetic field, this also, as a general rule, will result in an alteration in the slope of the negative portion of the characteristic. However, in this instance th change inthe slope of this part of the characteristic may be avoided by simultaneous modulation of one of the other quantities.

I claim:

1. In a magnetron oscillator tube and circuit arrangement therefor, where the tube possesses a linear cathode, a plurality of anode segments surrounding the cathode and a magnetic field exciter for controlling the direction offiow of electrons in the tube, the method of impressing modulation potentials upon the oscillatory output energy of said tube which comprises feeding said modulation potentials in push-pull to a pair of said anode segments, causing unequal biasing potentials to be appliedto said anode segments with respect to the cathodes, fixing the working point of the negative resistanc characteristic of said tube so that said bias potentials aresymmetric to a median value, and modulating said magnetic field exciter simultaneously with the impress of modulations upon the anode segsame, comprising an electron discharge tube having a linear cathode, a plurality of anode segments surrounding the cathode and a magnetic field producing means for influencing the flow of electrons in the tube, means under control of a source of modulating potentials for simultaneously varying the strength of the magnetic field in the discharg zone, and for opposingly varying the potentials applied to oppositely disposed anode. segments, and means for maintaining the anode potentials 'with respect to the cathode potential symmetric to a median value.

4. In an oscillator and modulating system therefor, where a magnetron discharge tube of the plural segment anode type is employed, the method of stabilizing the negative resistance characteristic of such a tube, so as to sustain oscillations therein, which comprises causing the magnetic field and the anode potentials to be simultaneously varied in response to modulating potentials applied thereto,- and causing a reduction of anode potential on .one anode segment with respect to the cathode potential to be symmetric to and coincident with an increment of 

